With the changing emissions requirements for mobile platforms or vehicles (e.g., automobiles, buses, trucks, etc.) alternative drive, such as fuel cell systems, and/or fuels, such as hydrogen, liquid natural gas, and propane are being implemented on the mobile platforms. The space available for storing fuel on these mobile platforms, however, may be limited. Thus, it is advantageous to maximize the storage capacity of a storage tank for these alternate fuels on the mobile platform.
To provide efficient storage of these alternate fuels, the fuels are stored in storage tanks at very low temperatures, such as 20 to 100° K., and at elevated pressures, such as 1–12 bars, to densify the fuel and thereby provide more of the fuel in a given volume of space. Such tanks are typically considered cryogenic storage tanks and may include a multi-layered vacuum super insulated cryogenic tank. The fuel stored in the storage tanks is typically maintained in a two-phase state of liquid and gas. As the fuel is stored, heat influx into the storage tank causes the temperature of the fuel to increase which also causes the pressure in the storage tank to increase. A pressure relief valve or vent valve is utilized on these tanks to vent the fuel from the tank when the pressure has risen to a predetermined level. The valves function to prevent the pressure within the storage tanks from exceeding that predetermined value.
The venting of the fuel is a loss of energy and, accordingly, increases the cost of operating the mobile platform on which the storage tank is utilized. Additionally, to avoid venting liquid fuel from the storage tank the tanks are operated in a manner that limits the liquid level of the fuel in the tank at all times to a maximum of 95% of the capacity of the tank. The vent valve draw off location from the storage is typically arranged at the 95% or higher level of the storage tank so that liquid fuel is not vented from the storage tank during a venting operation. Because the fuel is stored in a two-phase state of liquid and gas, at least the top 5% of the tank will contain the fuel in a gaseous state. The maintaining of at least 5% of the fuel in a gaseous state, however, does not maximize the capacity of the storage tank due to the lower density of the gas than the liquid fuel. Accordingly, it would be advantageous to utilize the entire capacity of the storage tank. Additionally, it would be advantageous to minimize the quantity of fuel vented from the tank during a venting operation while the fuel is being stored.